In this specification we are concerned particularly with methods of manufacturing axial flux permanent magnet machines. One known manufacturing technique for joining plastic is laser plastic welding. This has previously been used in the manufacture of low-tech mass produced parts—for example US2008/0261065 describes use of the technique in manufacturing an airtight housing for a pressure sensor. In some implementations of the technique, for example as illustrated in the abstract of JP2002/337236, one item to be welded transmits the (IR) laser light and the other absorbs the laser light, so that the laser reaches the joint. The joint, prior to welding, may comprise a rib formed on one or other item. It is known to measure the quality of the weld by measuring the collapse of such a rib, as described in LPKF Laser & Electronics 2011, “Quality Control” (downloaded from: http://www.laserplasticwelding.com/quality_control_impossibly_consistent.pdf)
An axial flux permanent magnet machine may function as either a motor or a generator, or both (at different times). Broadly speaking these have disc- or ring-shaped rotor and stator structures arranged about an axis. Typically the stator comprises a set of coils each parallel to the axis and the rotor bears a set of permanent magnets and is mounted on a bearing so that it can rotate about the axis driven by fields from the stator coils. FIG. 1a shows the general configuration of an axial flux machine with a pair of rotors R1, R2 to either side of a stator S—although a simple structure could omit one of the rotors. As can be seen there is an air gap G between a rotor and a stator and in an axial flux machine the direction of flux through the air gap is substantially axial.
There are various configurations of axial flux permanent magnet machine depending upon the arrangement of north and south poles on the rotors. FIG. 1b illustrates the basic configurations of a Torus NS machine, a Torus NN machine (which has a thicker yoke because the NN pole arrangement requires flux to flow through the thickness of the yoke), and a YASA (Yokeless and Segmented Armature) topology. The illustration of the YASA topology shows cross-sections through two coils, the cross-hatched area showing the windings around each coil. As can be appreciated, dispensing with the stator yoke provides a substantial saving in weight and iron losses, but drawbacks of removing the stator yoke are a) loss of the structural strength to the stator (which the iron provided) even though there is potentially increased need for strength because of the YASA topology which, being a compact design, can result in very large stresses and b) loss of a route for heat to escape from stator coils. To address both issues, i.e. the high torque density of the YASA design and generation of significant quantities of heat, a housing for the stator should provide great strength and rigidity to address torque demands and should also define a chamber which can be supplied with coolant for the machine. It can further be appreciated from FIG. 1b that for efficient operation (minimum losses in the high reluctance air gap) the gap between the rotor and stator should be as small as possible.
If the gap between the rotor and stator is very small, very tight tolerances are imposed on the dimensions of the stator housing, in particular the spacing and degree of parallelism in between radial end walls of the stator housing. The tolerance requirements are higher where there are two rotors, one to either side of the stator, and tolerance and alignment problems are particularly acute in some of the two-rotor designs we describe later.
It is generally desirable to improve the performance of axial flux permanent magnet machines. It is particularly desirable reliably to be able to manufacture axial flux permanent magnet machines with a very small rotor-to-stator gap.